Capillary study on geometrical dependence of shear viscosity of polymer melts

Abstract

Geometrical dependence of viscosity of polymethylmethacrylate (PMMA) and high density polyethylene (HDPE) are studied by means of a twin-bore capillary rheometer based on power-law model. Contrary geometrical dependences of shear viscosity are observed for PMMA between 210 and 255 C, but similar geometrical dependences are revealed for HDPE between 190 and 260 C. The fact that wall slip can not successfully explain the irregular geometrical dependence of PMMA viscosity is found in this work. Then, pressure effect and dependence of fraction of free volume (FFV) on both pressure and temperature are proposed to be responsible for the geometrical dependence of capillary viscosity of polymers. The dependence of shear viscosity on applied pressure is first investigated based on the Barus equation. By introducing a shift factor, shear viscosity curves of PMMA measured under different pressures can be shifted onto a set of parallel plots by correcting the pressure effect and the less shear-thinning then disappears, especially at high pressure. Meanwhile, the FFV and combining strength among molecular chains are evaluated for both samples based on molecular dynamics simulation, which implies that the irregular geometrical dependence of PMMA viscosity can not be attributed to the wall slip behavior.

Experimental investigation of the rheological behaviors of polypropylene in a capillary flow

“…They revealed that the entanglement–disentanglement slip mechanism was likely due to the geometrical viscosity of PMMA, but it was attributed to the adsorption–desorption slip mechanism for high‐density polyethylene. More recently, Lin et al . studied the geometrical dependence of the shear viscosity with a twin‐bore capillary rheometer.…”

Comparisons of the rheological behaviors of polypropylene and polymethyl methacrylate in a capillary die

“…As it is affected by the combination of the distinct temperature sensitivity of the viscosity and the relatively weak size effect, the PMMA viscosity clearly decreased as the temperature increased. Lin demonstrated the slip mechanism schematically in which there is an ultra‐thin melt film composed of entangled molecular chains between the polymer bulk and wall surface during a slip flow. The interaction force between the wall surface and the polymer chains determines the thickness of the lubricating layer.…”

“…It is known that increasing temperature is linked to the improved thermal motion of molecules and an increase in the free volume between molecules. A previous study demonstrated that temperature plays a significant role in the fraction of the free volume (FFV). The FFV of the polymer melt was analyzed by a molecular dynamics simulation (MDS) based on the N–P–T assembly system.…”

“…Considering that the fitted gradients of the of versus 1/ D curves are directly linked to the slip velocities, the negative slip velocities in the PP filling flow shown in Figure (a–c) imply that no wall slip occurred in the 0.5 and 1.0 mm dies at the tested temperatures. Wall slip only existed in the 0.25 mm die at 210 °C due as revealed by the positive gradients of the curves …”

Effect of feature spacing when injection molding parts with microstructured surfaces

“…Perfect venting conditions were assumed since the mold was vacuum vented. Although no wall slip and no pressure dependency of the viscosity are typical assumptions for injection molding simulations, these assumptions may result in deviations from molding results with thin wall features . The current version of the simulation package does not allow consideration of wall slip, but the pressure dependency of the viscosity was taken into account using the Cross‐WLF viscosity model.…”